1. Field of the Invention
The invention relates to circuitry for driving discharge lamps, and more particularly to circuitry for use in a liquid crystal display (LCD) backlight.
2. Description of the Related Art
There has been an ever-increasing demand for LCD displays within the past few years. Such displays are being employed by all types of computer devices including flat display monitors, personal wireless devices and organizers, and large public display boards. Typically, LCD panels utilize a backlighting arrangement which includes a discharge lamp that provides light to the displayed images. Among those currently available discharge lamps, cold cathode fluorescent lamps (CCFLs) provide the highest efficiency for backlighting the display. These CCFLs require high voltage AC to operate, mandating an efficient high voltage DC/AC inverter.
Although the operating voltage of the CCFL is typically of the order of some hundreds of Volts, a higher voltage is required initially to light up the CCFL. The lamp voltage required to ignite the CCFL is called the strike voltage or kick-off voltage. It is approximately 2˜3 times the CCFL operating voltage, for instance, the strike voltage may be up to 1500 volts. After applying the strike voltage, the CCFLs have some amount of delay time depending on their respective characteristics. In general, a CCFL inverter keeps on applying the strike voltage to the lamp for several seconds until discharge, and this period is commonly referred to as the ignition time. However, “open” or broken lamps can cause full voltage to appear at the output of a conventional CCFL inverter without overvoltage protection. For example, if a huge voltage, i.e., an overvoltage condition occurs, across the inverter's output terminals when the conventional CCFL inverter is turned on without the CCFL being in place, or when the CCFL becomes disconnected during normal operation due to a contact failure. This presents a safety hazard when touching or replacing the lamp. Further, the overvoltage condition can damage components of the CCFL inverter, and/or cause the inverter to run into an unexpected state, and eventually cause the inverter to be damaged.